1. Field of the Invention
This invention relates to apparatus for operating a gas discharge lamp, such as a fluorescent, a mercury vapor lamp, a sodium lamp, or a metal halide lamp.
2. Description of the Prior Art
Control circuits for gas discharge lamps are known which obviate the need for the usual heavy and expensive series ballast devices, corresponding to the inductor in this device. In such circuits, switching elements are provided to periodically switch the direction of current through the lamp to reduce the deterioration or errosion of electrodes, and to ensure a high enough frequency of switching to reduce the requirement for the size of the ballast. Such circuits generally require two switching elements for each direction of the current.
Attempts have been made to fabricate the same type of circuit using only a single switching element to cause current reversal on the lamp. For example, the U.S. Pat. No. 3,906,302, to D. B. Wijsboom, is directed to such an arrangement and incorporates an inductor in parallel with the lamp, which lamp is in series with a switching device. Such a switching device is generally operated at relatively high frequencies, such as 20 kHz. A significant disadvantage of this prior art device is that its control circuitry does not provide for varying the intensity of the lamp.
Such prior art circuits typically operate from a DC source, either from batteries or from a rectified and filtered AC source. In the latter instance, the filtering required results in a poor power factor, making the circuits unacceptable in certain applications.